Sludge disposal for many cities in the United States has become a problem of increasing dimensions in the last two decades. With the highest per capita sludge generation in the world, the U.S. now generates about 4.5 billion dry kg per year in sewage solids which must be disposed of or utilized in a manner posing the least amount of threat to the environment. At present, most of this sludge is disposed of by means of landfill, ocean dumping and incineration; relatively small amounts are applied to agricultural lands. In light of emerging environmental, energy and economic constraints, land application now appears to be one of the most viable sludge utilization and disposal alternatives for the future. However, present sludge management technology often cannot guarantee public health protection or cost effective solutions.
Among the known methods of disinfecting sewage sludge, heat treatment is one of the most effective and environmentally safe. Raising the temperature to 60.degree. C. or higher for several days greatly reduces the numbers of viable viruses and vegetative forms of bacteria and will assure the destruction of one of the most environmentally resistant organisms, ascaris eggs. The principal drawback of heat treatment is the necessary addition of external energy especially amid prospects of higher future energy prices and possible fuel shortages. It has been found, however, that thermophilic aerobic digestion has the capability of maintaining disinfection temperatures solely from the heat released from the biological oxidation of organic matter to carbon dioxide, thus requiring no supplemental heat addition. This concept was demonstrated by L. C. Matsch and R. F. Drnevich using a pure oxygen injection system, and is discussed in their article "Autothermal Aerobic Digestion", Journal Water Pollution Control Federation 49 (2), 296 (1977). While thermophilic aerobic digestion does not require the use of external energy, high operating costs result from the necessary addition of pure oxygen.
Studies in Europe have demonstrated autoheating by the use of self-aspirating aerators which increases the oxidation efficiency so that autoheating may be achieved without the addition of pure oxygen. These studies have not, however, applied the autoheating theory to municipal sewage sludges nor have they made use of insulated vessels to conserve the heat of oxidation whereby the digester may be maintained at thermophilic temperatures for extended periods of time.
While known sewage treatment facilities can produce pasteurized sewage sludge, the removal of heavy metals from municipal sewage sludge has seldom been emphasized as a necessary treatment step. The presence of heavy metals, particularly cadmium, at significant levels in sludges of municipal treatment plants receiving industrial discharges has become a topic of increasing concern in the light of the potential impact of heavy metals on the environment and the food chain. Within the decade, evidence has been accumulated indicating that cadmium in sludge-amended soils can be magnified and accumulated in the food chain. Of further concern, cadmium has been reported to accumulate in the human kidneys to life-time levels not far below concentrations that would be expected to produce damage to this organ. Other toxic heavy metals known to be present in municipal sewage sludge include chromium, copper, lead, nickel and zinc. It is well established that heavy metals can be removed from dilute wastewaters passing through a municipal treatment plant with a high degree of efficiency. In typical municipal treatment plants accepting effluent discharges from industry, heavy metals in the treated sewage can be either absorbed to organic particles and settled out in primary treatment or entrapped in the biological floc of the activated sludge process. Heavy metals can thus be removed from the sewage stream in primary and secondary treatment with combined efficiencies ranging from 60 to 90 percent in most cases. This results in a sludge with heavy metal concentrations 10 to 1000 times greater than concentrations measured in the influent sewage. The metals in the resultant sludge are usually predominant in their insoluble form as precipitates of hydroxide, carbonate, phosphate or sulfide, especially when heavy metal concentrations exceed 10.sup.-4 M and when pH values at or above neutrality are maintained. Sludges subject to prolonged thickening, storage, or anaerobic digestion, however, would have oxidation reduction potentials (ORP) in the range considered anaerobic, below -300 mv. Inorganic chemistry theory would predict that at low ORP and at near neutral pH, heavy metals such as cadmium, cooper, nickel, iron, lead and zinc exist at equilibrium primarily as insoluble sulfide precipitates.
The conversion of sludge heavy metals to the soluble form prior to physical separation would allow the removal of heavy metals with conventional dewatering techniques in use today (i.e., centrifugation, belt press, dewatering beds, vacuum filter, etc.).
For years, the extraction of heavy metals from sewage sludges and sludge/soil mixtures with acid washings has been used as a standard laboratory test to determine the extent of metal uptake likely to occur with plants grown on sludge-amended soils. Laboratory tests have been conducted on untreated and anaerobically treated sludge using acid extraction techniques to remove heavy metals. These studies have shown that the direct acidification of primary, waste-activated, or anaerobically digested sludge cannot consistently achieve a rapid and quantitative solubilization of heavy metals. While the data reported in the prior art seem to indicate that a processing scheme consisting of acid treatment combined with dewatering may be capable of some degree of removal of heavy metals from municipal sewage sludge, previous systems do not disclose the efficient, low-cost method of heavy metals solubilization by a combination of biological pretreatment and controlled-environment acidification.